The three books "Das Boot", "Die Festung" and "Der Abschied" are not fiction though, he just wrote what he experienced on different patrols, into one novel - maybe except the last "Der Abschied". Buchheim himself was not so pleased with the film "Das Boot" though, just because of some inaccuracies (the popping rivets being one of the well known technical glitches, the pressure hull was completely welded) and the conduct and presenting of the crew was a bit different, which is why the film was also disliked by some of the former crews.
Three more books bei Buchheim which are not fiction at all, full of photos from his time as a war reporter, and with a lot of information on the "real thing", from daily duty to technical stuff, and a lot of critic towards the then high command:
"Zu Tode gesiegt"
(there are some others, but mostly condensed material from those three)
You get them from time to time here in Germany, sometimes dirt cheap. Several books by Buchheim